The present invention relates to mercury dispensing compositions, as well as a manufacturing process thereof.
The compositions of the invention, thanks to their characteristics of stability in air and at low temperatures, and also of mercury release at high temperatures, are particularly suitable for the use in dosing mercury inside fluorescent lamps.
As known, fluorescent lamps require for their operation a gaseous mixture of noble gases at pressures of some hundreds of hectoPascal (hPa) and few milligrams of mercury vapor. In the past mercury was introduced into the lamps in liquid form, either by causing the same to drop directly into the lamp, or inside of small glass vials which afterwards were opened inside the lamp. However, due to the toxicity of mercury, the most recent international regulations have imposed the use of the lowest possible quantity of the element compatible with the lamps functionality; this has rendered the methods of liquid dosage obsolete, because these are not able to provide an exact and reproducible dosing in lamps of small quantities, up to about one milligram, of mercury.
Another method for the introduction of mercury into lamps is through the use of metal amalgams. The mercury release from these materials is however gradual, and starts already at relatively low temperatures, e.g. between 100 and 300° C., depending on the metal to which mercury is amalgamated. Because the manufacturing of lamps foresees operations that take place at relatively high temperatures when the lamp is not yet sealed, this results in the loss of a fraction of the mercury from the lamp and its release to the working environment; for example the sealing of the lamp is normally obtained by compression, under heating at about 500° C., of an open end thereof, and in this operation the amalgam can release to the outside a not negligible fraction of the initially contained mercury.
The applicant has proposed in the past various solid products which allow to overcome the problems seen before.
U.S. Pat. No. 3,657,589 discloses TixZryHgz compounds, which do not release mercury when heated up to about 500° C., but can release it when heated to about 800-900° C. (so-called activation treatment); the preferred compound of this family is Ti3Hg, sold under the trade name St 505. Compared to liquid mercury this compound has the advantage that it can be powdered and dosed into small weight quantities, for example by rolling the powders on a metallic strip with a known linear loading of mercury, and cutting from such a strip sections of the desired length, corresponding to the required weight of mercury. It has however been observed that the mercury release from such a material during the activation treatment is poor, between about 30 and 40% of the total mercury content; it is believed that the reason is an alteration of the material during the final operations of the manufacturing process of the lamps, during which the compound is exposed to oxidizing gases (air or gases released from the glass walls of the lamp itself during the heat sealing treatment). As a consequence, for a given quantity of mercury required by the lamp operation, the dosage by Ti3Hg requires the use of a quantity of mercury which is at least double or even three times, such a characteristic being in contrast to the stringent regulations mentioned above.
British patent application GB-A-2,056,490 discloses Ti—Cu—Hg compositions having better properties of mercury release compared to those of the compounds according to U.S. Pat. No. 3,657,589. In particular, these compounds are stable in air up to about 500° C., while by heating up to 800-900° C. they release quantities of mercury of more than 80%, or even up to 90%. However, these materials are characterized by a certain degree of plasticity, which makes difficult their milling. Since the manufacturing of devices containing these compounds, as well as the control of the uniform loading with mercury (linear in the case of strip or wire devices, per device in the case of discrete containers) requires the powdering of the compounds, these milling difficulties have in fact hindered the industrial use of these compounds.
The U.S. Pat. No. 5,520,560, U.S. Pat. No. 5,830,026 and U.S. Pat. No. 5,876,205 disclose combinations of powders of the compound St 505 with a promoter of the mercury yield (respectively, copper-tin alloys with possible additions of small quantities of other transition elements; copper-silicon alloys; and copper-tin-rare earths alloys); the addition of the promoter allows to increase the mercury yield from the compound St 505 up to values of 80-90%, even after its oxidation, thus solving the problem of the need of using a large excess of mercury, as resulting from the compound St 505 used alone. The use of a mixture of different powders raises however some problems in the manufacturing process of the devices containing the same: first of all, the two materials have different densities and rheological properties, and consequently they can separate from each other inside of the loading systems (e.g. the hoppers), causing thereby inhomogeneities in the mercury distribution. Furthermore, it has been found that, during the activation treatment, devices containing this mixture of powders may in some cases give rise to the ejection of powder particles of the promoter; although the phenomenon does not occur often and the ejected quantities are limited, this represents a problem in the manufacturing lines of the lamps.